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An iodine test is used to determine when the starch in grain has been fully converted into sugar by the mash.
How to perform an iodine test
An iodine test requires a white saucer, a small bottle of iodine with a dropper, and a small sample of liquid from the mash tun.
Let the liquid drip onto the saucer, being careful not to include any solid material such as grain husks, which could give a false positive reading. Then add a drop or two of iodine. If there is still unconverted starch in the mash, the wort will quickly turn dark blue or black.
Regardless of the color, do not return the sample to the mash tun.